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LM675: Inverting amplifier not amplifying AC component of input

Ismail Hegazi
Prodigy 70 points
Part Number: LM675
Other Parts Discussed in Thread: TLE2072,

Hi,

I have the circuit below, a wien bridge oscillator that generated a 15V 700 Hz signal - the oscillator works well on its own when not connected to any load.

When I load the amplifier with a small high power resistor (i.e. 50 ohms), the output of the oscillator dies down. Any ideas why that is happening?

  • +1
    TI__Guru* 93105 points

    Hi Ismall,

    The circuit should work in terms of the op amp configurations being applied. But do note that the TLE2072 produces a rail-to-rail ac square-wave output voltage that is about 1.5 V from each supply rail, while the OPA675 common-mode input voltage (Vcm) limits are about 3 V from each supply rail. Therefore, the LM675 Vcm is being overdriven and exceeded. It may tolerate this condition when its output is loaded lightly by the 1-kilohm feedback and input resistors, but when a 50-Ohm load is added to the output it stops functioning as an op amp.

    You should look at ways to limit the TLE2072 ac output so as not to exceed the Vcm of the OPA675. I do believe the various handbooks that cover the Wien Bridge show ways to add diode clamps to the stage, or adds a non-linear element to the oscillator circuit to limits its output amplitude. A properly limited Wien Bridge oscillator produces a sine wave instead of a rail-to-rail square wave - unless the latter is what you really want. Alternately, you could increase the LM675 power supplies so that its Vcm range is no longer exceeded.

    Additionally another point, all of the LM675 circuits in the datasheet have the output pin series RC network, 1-Ohm and 0.22 uF. I suggest including that network in your circuit to assure stability.

    Regards, Thomas

    Precision Amplifiers Applications Engineering